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Home , Bessel ellipsoid, Friedrich Robert Helmert, History of geodesy, Meridian arc, Paris Meridian, Toise

Journal of (2005) 78: 558–568 DOI 10.1007/s00190-004-0423-0

The International Association of Geodesy 1862 to 1922: from a regional project to an international organization W. Torge Institut fu¨ r Erdmessung, Universita¨ t Hannover, Schneiderberg 50, 30167 Hannover, ; e-mail: [email protected]; Tel: +49-511-762-2794; Fax: +49-511-762-4006

Received: 1 December 2003 / Accepted: 6 October 2004 / Published Online: 25 March 2005

Abstract. Geodesy, by definition, requires international mitment of two scientists from neutral countries, the collaboration on a global scale. An organized coopera- International Service continued to observe tion started in 1862, and has become today’s Interna- polar motion and to deliver the data to the tional Association of Geodesy (IAG). The roots of Central Bureau for evaluation. After the First World modern geodesy in the 18th century, with arc measure- War, geodesy became one of the founding members of ments in several parts of the world, and national geodetic the International Union for Geodesy and surveys in and , are explained. The (IUGG), and formed one of its Sections (respectively manifold local enterprises in central , which Associations). It has been officially named the Interna- happened in the first half of the 19th century, are tional Association of Geodesy (IAG) since 1932. described in some detail as they prepare the foundation for the following regional project. Simultaneously, Gauss, Bessel and others developed a more sophisticated Key words: Arc measurements – Baeyer – Helmert – definition of the ’s figure, which includes the effect – – of the field. In 1861, the retired Prussian general International Association of Geodesy J.J. Baeyer took up earlier ideas from Schumacher, Gauss, Struve and others, to propose a Central European in order to study the figure of the Earth in that region. This led to a scientific organization, which soon extended from Central Europe to the whole and later to the globe, and changed its name in 1 The origin of modern geodesy and first international 1886 to ‘Internationale Erdmessung’ (International Geo- cooperation: 18th century detic Association). The scientific programme also wid- ened remarkably from more local studies based on The beginning of modern geodesy may be reckoned geometric data to regional and global investigations, from the 17th century, when physics and with gravity measurements as an important source of postulated an oblate figure of the Earth (Perrier 1939; information. The Central Bureau of the Internationale Bialas 1982; Torge 2001). Based on the theory of Erdmessung was hosted at the Prussian Geodetic Insti- hydrostatic equilibrium, the flattening of a rotating tute in , and with Baeyer as Director, developed Earth was calculated, with values ranging between as an efficient tool of the Association. The scientific 1/230 (I. Newton 1687: ‘Philosophiae Naturalis Prin- research extended and deepened after 1886, when F.R. cipia Mathematica’) and 1/576 (C. Huygens 1690: Helmert became Director of the Central Bureau. A ‘Discours de la Cause de la Pesanteur’). This was a stronger international participation then took place, great challenge for geodesists: to prove the polar while the influence of the German states reduced. Of flattening by geometric methods and to determine the great practical importance were questions of standard- actual value of the flattening, which then would ization and reference systems, but first attempts to represent a boundary value for physical Earth models. interpret gravity field variations and to monitor geody- It started the period of arc measurements at different namic phenomena by geodetic methods indicated future , whereby latitude arc measurements prevailed tendencies. With the First World War and the expiry of in practice. Here, the length of a part of a is the last international convention in 1916, the interna- determined by triangulation, a method which had been tional cooperation within the frame of the Association successfully introduced by the Dutchman Snellius at the practically came to an end, which ended the first epoch of beginning of the 17th century, and which was employed the Association. Nevertheless, due to the strong com- for nearly 300 years. By measuring the corresponding 559 difference of the latitudes using well-known astronom- set an example for international collaboration. It also ical procedures, the Earth’s curvature at that latitude demonstrated a fruitful cooperation between scientists could be calculated. and military state organizations, which were soon found Naturally, the solution to this problem required in other countries. During this time, there were signifi- international collaboration. A driving force in this cant improvements in the measurement and computa- direction was the French Academy of Sciences, founded tion techniques, characterized by the introduction of the in 1666, which emphasized the importance of the prob- Borda repetition circle and baseline apparatus in France, lem of the Earth’s figure. First attempts to calculate the the Ramsden second in Great Britain, and flattening from measurements carried out in France mathematical developments for the calculation of tri- along the meridian through by J.D. Cassini and angulation networks on the by Legendre, others (around the turn of the 18th century) failed, and Lagrange, Laplace and others. the French Academy consequently initiated the well- known arc measurements in Lapland (1736–1737) and in the Spanish Vice-Kingdom of Peru (today ) 2 Collaboration in central Europe in the 19th century: (1735–1744). These enterprises required political agree- the forerunners ment between the countries involved, but, through the participation of non-French scientists and engineers, The political situation in central Europe was less favour- these French expeditions obtained an international able in the 19th centuary, due to the strong division into character. The Lapland expedition, led by the French local territorial units, which was especially pronounced in Academy member Maupertuis,1 included the Swedish Germany. On the other hand, this diversity led to a variety astronomer Celsius. In addition to the French acade- of different solutions for geodetic surveys, and several micians Godin, Bouguer and La Condamine, the interesting attempts to contribute to the determination of Spanish officers Jorge Juan and par- the size and shape of the Earth. As the organized ticipated in the measurements and the evalution of the international cooperation in geodesy started in central Peru expedition. Europe during the second half of the 19th century, we next The early combination of the Lapland arc measure- consider the developments that led to this step (Torge ment with the revised Paris then confirmed 1997). the oblate ellipsoidal Earth model, with a flattening of There were a number of state surveys in the last 1/304, which is close to the value known today decades of the 18th century, which were based on the (1/298.25). Further arc measurements in the 18th cen- method of triangulation, such as the survey of Denmark tury delivered, by different combinations and calculation (1764–1792) under the direction of Thomas Bugge, and procedures, flattening values of between 1/144 and the survey of the principality, and later kingdom, of 1/352. Gravity measurements, although carried out in Saxonia in Germany (1780–1811), under Friedrich connection with most of the arc measurements, were not Ludwig Aster. On the other hand, in the survey of the then exploited for the determination of the Earth’s principality of Hannover (1764–1786) under Josua de flattening according to Clairaut’s theorem (published in Plat and Johann Ludwig Hogrewe, plane table mea- 1743 in ‘The´ orie de la Figure de la Terre, tire´ edes surements were only locally connected by short base- Principes de l’Hydrostatique’). However, the common lines, and orientated by a few astronomically derived adjustment of only 19 measurements performed until positions. The situation was even worse in the kingdom 1760 would have led to a reasonable flattening value of of Prussia, which covered a large part of northern 1/328 (Ekman and Ma¨ kinen 1998). Germany. Topographical information was kept secret With the strengthening of the European national for military reasons, and mapping of larger areas uti- states, and the related changes in economy, adminis- lized existing local information, supplemented and tration and military organization, the 18th century also connected by occasional plane table surveys. hosted the first national geodetic surveys, organized or A radical change happened at the turn of the 19th sponsored by the respective national governments. century, with the extension of French influence over These surveys provided a network of geodetic control central Europe through the Napoleonic wars. The geo- points, which then served as a basis for topographical detic tradition built up by the French arc measurements and partly also cadastral maps, as well as for large-scale and the triangulation of France, but also the establish- engineering projects. France was the leading nation, ment of a land recording system () on the ini- with a country-wide triangulation for the ‘Carte tiative of Napoleon, strongly influenced the German ge´ ome´ trique de la France’ (Cassini de Thury, 1733– states. The French military surveys, which were per- 1750). The connection between the astronomic obser- formed between 1801 and 1813, played a special role in vatories in Paris and Greenwich (1784–1787) was the the states occupied by or allied with France. These beginning of the national geodetic survey of Great surveys were based on a triangulation, and connected to Britain under the direction of the , and the French triangulation and the recent arc measure- ment through the by Delambre and Mechain (1792–1798), which was carried out in order to 1It should be mentioned that in 1741 Maupertuis was appointed as President of the Academy of Sciences in Berlin by Frederic II the define the as a natural unit of length. Great, King of Prussia, which is another example of the interna- In Germany, the first half of the 19th century was tional character of science in those times of absolute monarchies. characterized by triangulations carried out in most 560

German states as the basis for mapping. The persons in the Danish astronomer Heinrich Christian Schumacher, charge of these operations were either scientists (mainly who was in charge of a new triangulation of Denmark, astronomers and mathematicians) or military officers and who suggested to Professor Gauss in Go¨ ttingen the with scientific interest and skills. As a consequence, there extension of the triangulation southwards as part of a were also several attempts to use these state surveys for central European arc measurement. The reconnaissance, the determination of the figure of the Earth. A first measurement and calculation of the arc between Altona outstanding result was the triangulation of Bavaria (the seat of Schumacher’s astronomical observatory) starting in 1808, which followed the French military and Go¨ ttingen were undertaken personally by Gauss, survey. It is connected with the name of Johann Georg where he introduced the method of least-squares (LS) von Soldner, who introduced new methods of calcula- adjustment. The arc measurement was connected to the tion. By using new survey instrumentation developed by Dutch triangulation, and hence to the French meridian the Bavarian workshop of Reichenbach, the accuracy of arc in the west, to the triangulations of Mu¨ ffling, and to the results was significantly increased. This was a pre- the state of Hessen (organized by Christian Ludwig requisite for the purpose of the survey, namely the Gerling, a student of Gauss) in the south. A further production of large-scale cadastral maps, which should extension south failed because the results of the trian- also provide the basis for topographical mapping. In gulation of Bavaria were not made available, and large- north-western Germany, a military triangulation was scale geodetic control in Italy did not yet exist. However, carried out by the Prussian colonel Karl Ludwig von the vision of Gauss was already that ‘‘... perhaps it is Lecoq (1796–1805), in a period when Prussia had with- not an unrealizable prospect, that one day all the drawn from the coalition against France. South of this astronomical observatories of Europe could be con- area, a triangulation of Thuringia started in 1803, under nected together by trigonometric means ...’’ (Gauss the direction of Franz Xaver von Zach. 1828). This survey was to be extended to an arc measurement From the theoretical point of view, the first decades in central Germany, but was interrupted by revival of of the 19th century were also of great importance for the war between Prussia and France. geodesy, because Gauss, Bessel and others refined the At the end of the Napoleonic wars, Prussia reorga- definition of the figure of the Earth by taking the gravity nized its mapping organizations and concentrated them field into account. There was now a clear distinction on the general staff (see e.g. Torge 2002). Its chief, between the physical surface of the Earth described by Friedrich Carl Ferdinand Freiherr von Mu¨ ffling, the topography, the equilibrium surface of constant immediately organized a first-order triangulation that gravity potential coinciding with the surface of the started from the French military surveys along the oceans (later called the ), and the ellipsoid as a Rhine River and ran over Berlin to Silesia (1817–1828), mathematically simple reference surface. with later extension to eastern Prussia and connections The subsequent Bessel–Baeyer period of the Prussian to the Russian triangulation chains along its western state survey finally prepared the organized international boundary. Those surveys had started in 1816 under the cooperation in that region (Dick 1994, 1996). It was direction of the astronomer Friedrich Georg Wilhelm triggered by the Russian proposal of 1829 to connect the Struve and the general Karl Ivanovic von Tenner, and triangulation in the Baltic States (Wilhelm Struve) were intended to also serve as a meridian arc of great with the Prussian triangulation chains established by length. Von Mu¨ ffling had already participated in the Mu¨ ffling and his successors. Friedrich Wilhelm Bessel, earlier triangulation in north-western Germany and in astronomer in Ko¨ nigsberg, Eastern Prussia, strongly Thuringia, and was highly interested in establishing a promoted this proposal and widened it to a proper arc arc measurement between the Paris meridian measurement intended to connect the Russian triangu- arc (for this purpose he contacted Delambre in 1815, lation with the manifold state surveys and arc mea- when he was governor of Paris, which was occupied by surements in central and western Europe. From the allied troops) and the existing or developing triangula- Prussian general staff, the experienced major Baeyer tion networks in Germany, as a contribution to the was detached from this project (1831–1836), which by determination of the figure of the Earth. Although this improved methods of observation and evaluation set project failed, the systematic organization of the new standards for geodetic work. After this, Baeyer was Prussian state survey under von Mu¨ ffling and his interest rapidly promoted and finally led the trigonometric in geodetic problems was the school for Johann Jacob department of the general staff (1843–1857). Baeyer, who later became the founder of the ‘Central Under his direction, improved triangulation chains European Arc Measurement’ as a scientific organiza- were spread out over Prussia, with proper connections tion. He performed topographic and trigonometric to the neighbouring countries. For various reasons measurements under von Mu¨ ffling, assisted in calculat- (among them the colliding opinions on the mapping of ing the ‘Mu¨ ffling ellipsoid’ as the reference surface for the country, whereby Baeyer 1851 unsuccessfully pro- the calculations, and entered the general staff in 1821, posed to produce a large-scale map as a basis for civil where he began a rapid career progression (described and military purposes, and the appointment of a youn- below). ger officer, Helmuth von Moltke, as chief of the general An outstanding enterprise of this period was the arc staff), general Baeyer retired from the general staff in measurement of the kingdom of Hannover, carried out 1857, at the age of 63. He then concentrated on scientific by (1821–1824). It was initiated by work on refraction, and continued his engagement in the 561 determination of the figure of the Earth through dis- cussions with neighbouring countries, and initiated new measurements supported by the general staff. These years arguably prepared him for his second ‘career’ as the founder and organizer of an international geodetic project.

3 Foundation and consolidation — the ‘Baeyer’ period: 1862–1886

During his collaboration with Mu¨ ffling, Struve and Bessel, Johann Jacob Baeyer (1794–1885) (Fig. 1) had recognized the importance of large-scale astrogeodetic systems for the determination of the figure of the Earth. After his retirement he concentrated on this problem, influenced also by his appointment as Prussian repre- sentative to the longitude arc measurements at 52 Fig. 1. Johann Jacob Baeyer (1794–1885), founder of the latitude, which was proposed by Wilhelm Struve, then ‘Mitteleuropa¨ ische Gradmessung’ and President of the Central Bureau director of the Dorpat astronomical observatory (1864–1885). Courtesy GFZ Potsdam (Buschmann 1994). In April 1861, Baeyer presented a ‘Proposal for a Central European Arc Measurement’ (‘Entwurf zu einer mitteleuropa¨ ischen Gradmessung’; Fig. 2) to the Prussian Minister of War, and justified this project by a comprehensive memorial on the size and figure of the Earth, which he dedicated to the memory of Alexander von Humboldt (Baeyer 1861). After being commanded to the trigonometric bureau of the general staff in Berlin in 1823, Baeyer had been introduced to Alexander von Humboldt. This famous naturalist acknowledged the geodetic abilities of Baeyer, and even proposed his participation in the planned expedition to central Russia and Siberia. Although this cooperation did not come off, Humboldt continued to observe Baeyer’s career. In a letter directed to the King of Prussia in 1837, Humboldt characterized Baeyer as one of the most experienced officers who could be found in any army. In 1854, he prepared a positive evaluation of Baeyer’s rather radical (but not successful) proposal of 1851, to reorganize the activities in Prussia on a higher level and to centralize them. When Baeyer’s return to the practical military service (as chief of a brigade) was planned in 1856, Humboldt argued ‘‘the King of Prussia owned sufficient officers for commanding a brigade, but only one Baeyer’’. The objective of Baeyer’s 1861 project was the determination of anomalies in the Earth’s curvature (i.e. deflections of the vertical, and thus the relative structure of the geoid) in central Europe, to be achieved by col- lecting and combining existing data and performing new measurements, taking high quality standards into account (Torge 1994). Baeyer clearly described the sci- Fig. 2. Hand-written draft (first and last lines) of Baeyer’s memo- entific problem to be tackled, which was not only to randum ‘Entwurf zu einer Mitteleuropa¨ ischen Gradmessung’, pre- determine the deviations of the Earth’s figure (namely sented to the Prussian authorities in April 1861, from Dick (1994) the geoid) from the ellipsoidal model, but also to inter- pret these anomalies by the structure and composition of the outer layers of the Earth. In other words: the thus- far discussed pure geometric problem of determining the which required intensified interdisciplinary cooperation; parameters of an ellipsoid from arc measurements was the future science discipline ‘physics of the solid Earth’ extended to a more general problem of natural sciences, became visible. 562

The memorandum in Fig. 2 describes the great arc measurements in western (Paris meridian) and eastern (Dorpat meridian) Europe and emphasizes the excellent conditions for corresponding investigations in central Europe, namely a multitude of astronomic observato- ries and local triangulations, which only had to be put in order, and eventually improved and connected by additional measurements, in order to obtain a large- scale geodetic network (Fig. 3). On 20, June 1861, Baeyer’s plan was approved by order of the Prussian royal cabinet. First negotiations between representatives of the states of Prussia, Austria and Saxony took place in Berlin from 24 to 26 April 1862 (Levallois 1980). The participants of this first meeting were as follows: J. J. Baeyer, Lieutenant-General z.D., Prussia A. von Fligely, Major-General and Director of the Military-Geographic Institute Vienna, Austria C. von Littrow, Director of the Astronomic Observa- tory Vienna, Austria J. Herr, Professor for Spherical Astronomy and Higher Geodesy, Polytechnical School Vienna, Austria J. Weisbach, Professor at the Royal Montanistic Academy , Saxony C. Nagel, Professor of Geodesy, Polytechnical School , Saxony C. Bruhns, Professor of Astronomy, University Leipzig, Saxony. At the end of 1862, Baeyer was able to identify 16 states or countries that had entered the project: Austria, Belgium, Denmark, France (allowed the use of data necessary for the project), seven German states (Baden, Bavaria, Hannover, Mecklenburg, Prussia, Kingdom of Saxony, Saxe-Gotha), Italy, The , Poland (through Russia), Sweden and Norway (in personal union), and Switzerland. This was a great success: an international scientific (governmental) organization had been established within a remarkably short time. Among the representatives were well-known scientists, but also experienced military officers, as the national surveys in Fig. 3. Network sketch of the proposed ‘Central European Arc most of the countries were then in the hand of military Measurement’, including astronomical observatories and connecting agencies. lines to be calculated from triangulation nets, from Baeyer’s April Already at this early stage, the scientific programme 1861 memorandum of the project was defined in more detail and slightly extended. In addition to the proper arc measurements (selection of the astronomical points and connection of took place in Berlin, which we might consider as the the existing geodetic networks by additional triangula- forerunner of the General Assemblies of the IAG and tions, and establishment of more baselines in order to IUGG. At this conference, the organizational structure strengthen the scale of the meridian arc), the questions (Permanent Commission with annual meetings being of the (the 1841 was responsible for the scientific management, Central introduced), the length standard (comparison of the Bureau as an executive, General Conferences at three- different standards used in the participating countries), yearly intervals) was fixed, as well as the research and the required accuracy (definition of an error limit programme. The scientific recommendations of this first for the inclusion of older triangulation) were discussed. conference included the following. In addition, the data sources to be exploited were ex- tended to include gravity measurements, 1. Bessel’s should be used as the length unit for the which introduced the physical component into the pre- geodetic calculations. viously geometrics-only problem. 2. All measures must be compared with Bessel’s toise. In 1864, the first ‘General Conference of the Repre- 3. The relationships between all units in use and the sentatives to the Central European Arc Measurement’ metre should be determined. 563

4. After the determination of the relationship between Bessel’s toise and the metre, all results should be published in and in . 5. In addition to the trigonometric determination of heights, first order levelling should be carried out in all countries participating in the Central European Arc Measurement. 6. The heights in each country should be referred to a single zero point, and all these points should be connected by precise levelling. 7. The mean sea level of the various seas should be determined preferably by recording devices, and the zero points of the tide gauges should be included in the levelling. 8. As a result, a general height system for Europe Fig. 4. Building of the former Geodetic Institute Potsdam, seat of the should be selected. Central Bureau of the International Geodetic Association and its forerunners from 1892 to 1921 and now part of the Geoforschungs- The Central Bureau began its work in 1865 (financed zentrum Potsdam (GFZ). Courtesy GFZ Potsdam by Prussia), with Baeyer as President. In 1867, after Portugal, and Russia joined the association, the name of the organization was changed to the ‘Euro- pean Arc Measurement’. At this 2nd General Confer- Prussia allied with Austria), Prussia and Austria (1866, ence, the importance of gravity measurements was Austria allied with several German states), and Prussia emphasized, and a recommendation reaching far together with the other German states and France beyond geodesy was passed, namely that to establish (1870–71), which finally led to the unification of the an international Bureau for Weights and Measures and German states to a German empire under Prussian to produce a new metre standard. Also in 1867, Baeyer leadership. applied to create a Prussian Geodetic Institute. In 1870, We now briefly describe some of the investigations this institute was established in Berlin, and entrusted and results obtained in the first epoch of the organized with the operation of the Central Bureau. In 1892, it geodetic operations in Europe (Lambert 1950; Vo¨ lter moved to a specially constructed building located on 1963; Torge 1996). the Telegraphenberg in Potsdam, which today hosts the According to the project definition, primarily hori- geodetic section of the GFZ, the German Geo- zontal geodetic control data (triangulation, baselines) Research-Center (Fig. 4). were collected. The number of triangulation points in The General Conferences held in the two decades of Europe increased from 2010 in 1862 to 5546 in 1912, and the Baeyer period took place in cities in Germany and around 100 baselines were available at the start of the Austria: Berlin (1864 and 1867), Vienna (1871), Dresden project. Connected to astronomical observatories, and (1874), Stuttgart (1877) and (1880). A highlight with many intermediate astronomic latitude and longi- for the organization, and for Bayer’s work, was the tude determinations, these networks delivered deflec- conference in Rome (1883). Baeyer, then at the age of tions of the vertical. A remarkable enterprise was the 89, could not attend, but received a gold medal specially connection of the Spanish triangulation with dedicated to him by the Italian Arc Measurement (1879), where triangles with a maximum side length of Commission. Although this first period of the Associa- 270 km were observed from stations, under tion was governed by the outstanding personage of the direction of General Iba´ n˜ ez and Major F. Perrier, Baeyer, the work of the presidents of the Permanent Chief of the geodetic section of the French general staff. Commission should also be acknowledged; these were as (His son, General Georges Franc¸ ois Perrier, became follows. Secretary General of the IAG in 1922 and kept this office until his death in 1946.) 1. 1864 –1868: Peter Andreas Hansen (1795–1874), It is odd that the results of the Prussian Geodetic Director of the Gotha Observatory, Thuringia. Survey, which had been led by Baeyer for a long time, 2. 1869–1874: August von Fligely (1810–1879), General, were not accepted for the arc measurement project by Director of the Military-Geographic Institute, Vien- him. He accused the State Survey of making too many na, Austria. errors and of not keeping the accuracy rules necessary 3. 1874 –1886: Carlos Iba´ n˜ ez e Iba´ n˜ ez de Ibero (1825– for the arc measurements. This led to a confronta- 1891), General, Director of the Geographical and tion between Baeyer and the Geodetic Survey, which Statistical Institute, Madrid, Spain. involved a number of scientists and lasted until Baeyer’s It is remarkable that the Central European Arc Mea- death. On the other hand, as the Prussian army was not surement and the European Arc Measurement organi- too popular in the countries around Germany, due to zations developed so rapidly in the 1860s, eventhough the wars that led to the unification of Germany in 1871, this period was characterized by a number of wars in Baeyer certainly gained a lot of sympathy, especially central Europe: between Prussia and Denmark (1864, with the military representatives of those countries. 564

Large-scale first-order geometric levelling started in At the 1883 Conference, a first step to observing most European countries, following the experience the Earth as a dynamic system in space was recognized. gained from France and Switzerland. It soon became the The Italian astronomer E. Fergola proposed to monitor standard procedure for precise height determination, the location of the Earth’s rotation axis with respect to and completely superseded the trigonometric method for the solid Earth by latitude observations on the same the establishment of national height systems. Recording parallel. In 1884–85, F. Ku¨ stner observed the predicted tide gauges were installed at all seas surrounding latitude changes in Berlin, which led to the systematic Europe, and connected to the first-order levelling investigation of polar motion starting in 1889. networks. This carries us to the next period of the ‘Internatio- In order to increase the number of gravity stations, nale Erdmessung’, characterized by increasing interna- the Repsold workshop in Hamburg was asked to con- tional participation, by the extension of the problems to struct a transportable reversible pendulum apparatus. A be tackled, and by the outstanding figure of F.R. limited number of gravity measurements were carried Helmert acting as Director of the Central Bureau. out by this method until about 1900, but the results were not satisfactory. This was due — among other reasons — to the effects of co-oscillation (detected and 4 Extension and deepening — the ‘Helmert’ period: investigated by C.S. Peirce from the US Coast 1886–1916 and Geodetic Survey) between the pendulum, its support and the ground, and led to a multitude of investigations Baeyer died in 1885, and the first General Conference on the theory of the reversible pendulum. after his death, held in Berlin in 1886 with Wilhelm The General Conference had already chosen the Foerster (1832–1921), Director of the Berlin Astronomi- in 1867. The International Metre Con- cal Observatory as Chairman, brought a new convention vention of 1875 then finally solved the early-recognized on the organization, which was called ‘Internationale problem of the different length units in use, and estab- Erdmessung’ (‘Association Ge´ ode´ sique Internationale’ lished the International Bureau for Weights and Mea- in French, translated into English ‘International Geo- sures in Se` vres near Paris. With a new stable metre detic Association’). Until 1889, the , standard and copies distributed to the countries that had Mexico, Chile, Argentina and Japan agreed with the signed the , the same length unit was convention, and Great Britain joined the Association in now available in all countries. (Prussia introduced the 1898. Following the proposals made by Prussia, an metre in 1870, leaving the ‘Bessel foot’, which had been annual financial contribution from the countries was derived from the length of a second-pendulum by Bessel, provided to the Permanent Commission and the Central after very careful investigations between 1835 and 1837.) Bureau, and the Director of the Central Bureau and a Triggered by the geodetic activities of the Arc Permanent Secretary became responsible for the perfor- Measurement organization, H. Bruns, Professor of mance of the scientific and administrative work of the Mathematics at the University of Berlin, published a Permanent Commission. Voting at the General Confer- study on the fundamental problem of geodesy, which ences now followed the principle of one voice per looked far into the future, and described consequences country, which reduced the overwhelming influence of for the programme of the organization (Bruns 1878). the German states. The tendency of reducing individual Defining the determination of the Earth’s gravity po- influences and strengthening the international basis, as tential as the principal task of geodesy, he developed a well as the independent position of the Central Bureau, three-dimensional model for the solution of this problem was pursued at the renewal of the convention in 1895 that employed all available astronomic, geodetic and and 1906. The more international character of the gravimetric observations. However, it took nearly a Association can be seen also from the locations of the whole century before this approach of solving the figure General Conferences: Paris (1889), Brussels (1892), of the Earth without any hypothesis and without the Berlin (1895), Stuttgart (1898), Paris (1900), Copenha- introduction of an ellipsoid could be successfully taken gen (1903), Budapest (1906), London and Cambridge up in geodesy, using space techniques as the main tools. (1909), and Hamburg (1912). In 1882, the Senate and the Geographic Society of the Under the new conventions, the elected presidents city of Hamburg asked the Permanent Commission to were (Figs. 5–7) as follows: deal with the unification of the geographic by selecting one zero meridian and to suggest a corre- 1887–1891: Carlos Iba´ n˜ ez e Iba´ n˜ ez de Ibero, who al- sponding decision. This question was discussed at the ready served as President of the Permanent Com- General Conference in Rome in 1883, which was also mission attended by observers from Great Britain and the Uni- 1892–1902: Herve´ Etienne Auguste Albans Faye (1814– ted States as countries extremely interested in this 1902), President of the Bureau des Longitudes, Paris problem. The Conference decided to select the Green- 1903–1917: Jean Antonin Le´ on Bassot (1841–1917), wich meridian as the zero meridian for longitude, with General, Chief Geodetic Section, Service Geographi- the Universal Time referred to it. At the 1884 Meridian que de l’Arme´ e/Director Nice Observatory. Conference in Washington, a general agreement on this The Permanent Secretaries were as follows: definition was obtained, and gradually all countries 1886–1900: A. Hirsch, Director of the Neuchatel referred their longitudes to the Greenwich meridian. Observatory, Switzerland 565

1900–1921: H. G. van de Sande Bakhuizen, Director of the Leiden Observatory, The Netherlands. This period of the International Geodetic Association was strongly influenced by (1843–1917) (Fig. 8), who became Director of the Central Bureau in 1886–87 (Wolf 1993). Helmert studied surveying and geodesy, mathematics and physics at Dresden and Leipzig, Germany, and took part in the first-order triangulation of Saxony under Professor Nagel. He held the chair of geodesy at the Technical University of between 1870 and 1886, and at that time wrote the book ‘Die mathematischen und physikalischen Theorien der ho¨heren Geoda¨sie’ (1880–84), which arguably established geodesy as a proper science. From 1887, he held the chair of higher geodesy at Berlin University, and was Director of the Prussian Geodetic Institute, which continued to host the Central Bureau of Fig. 7. General Jean A. L. Bassot (1841–1917), President of the the Association. International Geodetic Association 1903–1917, from Levallois (1980)

The main achievements of Helmert’s period are as fol- lows. The collection of horizontal control data continued and extended over the Americas, Asia and Africa. Na- tional Reports regularly gave the geodetic progress in the participating countries. In 1912, a total of 9211 triangulation points were recorded worldwide, as well as numerous astronomic latitude, longitude and azimuth determinations. Extended meridian and parallel arcs were formed from these triangulations, and deflections of the vertical were derived and published by the Central Bureau. Of note were the re-measurement of the classical Peru arc by French geodesists (1899–1906), the (fourth) measurement along the Paris meridian with extension to Spain and Algeria (1870–1894), the 39 parallel arc mea- surement across the United States (1871–1898), the Afri- can latitude arc measurement along the 30 meridian, Fig. 5. General Carlos Iba´ n˜ ez e Iba´ n˜ ez de Ibero (1825–1891), starting in 1883 and completed in the 1950s, and a President of the Permanent Commission and of the International Geodetic Association 1874–1891. Courtesy Instituto Geogra´ fico Nacional, Madrid

Fig. 8. Friedrich Robert Helmert (1843–1917), Director of the Fig. 6. Herve´ E. A. A. Faye (1814–1902), President of the Interna- Central Bureau of the ‘Internationale Erdmessung’ (International tional Geodetic Association 1892–1902, from Levallois (1980) Geodetic Association) 1886–1917, from Wolf (1993) 566

Russian/Swedish latitude arc measurement in Spitzber- the development and extensive use of the torsion balance gen (1898–1902). The triangulation data were used not by R.v. Eo¨ tvo¨ s (since 1890) offers another method for only for the determination of best-fitting , but the determination of local gravity field structure, which also for investigations on isostasy, the outstanding gained great importance for oil exploration in the 1920s. example being the adjustment of the United States trian- The development of a horizontal pendulum by gulation net by O.H. Tittmann and J.F. Hayford. E. v. Rebeur-Paschwitz allowed the first observations of Precise levelling continued in many countries, and by Earth tides (1889–1893), with more regular measure- connecting national networks and ties to tide gauges, a ments carried out in Potsdam by Hecker (since 1910). united European levelling network was constructed. It is here that W. Schweydar (1914) observed the After first evaluations of the mean sea level records gravimetric Earth tides, and there were even first (periodic parts, secular trend), it was found that mean attempts (1909) to establish a global observing system sea level around Europe does not belong to one equi- for studies of crustal movements, jointly with the potential surface, but that deviations remain less than International Association for Earthquake Research. 0.2 m. The discussion on a common height datum for The determination of polar motion by astronomic Europe started, but the problem could only be solved at latitude observations was continued in Helmert’s period, the end of the 20th century, and is still open in other and put on a solid scientific basis. Observations in regions and globally. From the repeated levelling in Berlin, Potsdam and Prague (1889–90), and the results France (25 years’ repetition time), height changes were of an expedition to Honolulu (1891–92, A. Marcuse) found and were discussed in connection with suspected with parallel observations in Berlin, clearly show the recent crustal movements. Although these first inter- ‘Chandler’ period of about 427 days, detected by S.C. pretations were wrong, they formed another step to Chandler in 1891, and interpreted by S. Newcomb as the considering the Earth’s surface as time dependent, and Euler period lengthened through the Earth’s elasticity to the later formulation of four-dimensional geodesy. (1893). In 1888, W. Foerster took up the question of The number of observed gravity values increased sig- establishing an international observing system along one nificantly in Helmert’s period, due to the development of a parallel. In 1899, the International Latitude Service relative pendulum apparatus by R. van Sterneck in 1887. started regular observations at Mizusawa, Japan About 1400 relative gravity measurements were available (Director H. Kimura), Carloforte, Italy, Gaithersburg at the start of the 20th century, but they referred to dif- and Ukiah, USA, all located on the 39 08¢ parallel. ferent absolute values. This raised the problem of a Equipped with specially designed , the common gravity reference system, which was solved by observatories at Tschardjui (later moved to Kitab, a carefully executed absolute determination at the Russia, now Uzbekistan) and Cincinatti, USA later Geodetic Institute at Potsdam (Ku¨ hnen and Furtwa¨ ngler, joined this service. More observatories followed during 1898–1904), and the connection of all other gravity mea- the next decades, and this early international observing surements to the Potsdam value. The ‘Potsdam Gravity programme was only replaced and extended in 1988, System’ (presented by E. Borrass) was adopted in 1909 by the International Earth Rotation Service (IERS). (using more than 2400 gravity values), and served as The evaluation of the earlier latitude observations was gravity standard in , geodesy and geophysics performed at the Central Bureau in Potsdam, and until 1971. First gravity measurements on the oceans were continued during the First World War (Ho¨ pfner 2000). performed by O. Hecker between 1901 and 1909, and were This brings us to the end of the ‘Helmert’ period, with used for the investigation of the Eo¨ tvo¨ s effect, as well as the expiry of the international convention at the end of for obtaining a first impression of the isostatic behaviour 1916, and the practical end of most activities due to the of the water-covered areas of the globe. start of the First World War in the autumn of 1914. Theoretical studies and the evaluation of the astro- geodetic and gravimetric data collected and collated by the Central Bureau also brought significant progress in 5 Survival and transition to the IUGG section of geodesy: gravity field modelling. While large-scale evaluation of 1917–1922 the deflections of the vertical failed due to interpolation problems between the scarce observations, local test The convention on the ‘Internationale Erdmessung’ areas such as the Harz in Germany provided expired at the end of 1916, and was not extended due insight into the gravity field structure, including the local to the First World War. In addition, some of the leading geoid behaviour (Helmert, Galle). Much effort was spent officers of the Association died during these years, on investigations of the ‘reduction’ of gravity field data among them the President, Bassot, from France (1917), to sea level, and we see the beginning of the interpreta- the Vice-President, Sir George Darwin, from Great tion of gravity anomalies with respect to crustal Britain (1912) and his successor, O. Backlund, from structure and isostasy (Faye, Hayford, Helmert). The Russia (1916), and the Director of the Central Bureau, increased number of gravity values allowed calculation Helmert, from Germany (1917). of improved values for the normal gravity field and the It is remarkable that some activities of the Associa- flattening of the reference ellipsoid (formulas by Helmert tion still continued, due to the efforts of two men in 1884 and 1901), which soon served as reference for from neutral countries, Raoul Gautier (1854–1931), many practical surveys. First estimates (1910) on the Director of the Geneva Observatory, Switzerland, and global behaviour of the geoid stem from Helmert, and Hendrikeus Gerardus van de Sande Bakhuyzen (1838– 567

1923), Director of the Leyden Observatory, The Neth- the ‘Mitteleuropa¨ ische Gradmessung’ there were re- erlands, and Secretary of the Association since 1900. ports on the origin and development of the Associa- They proposed that the neutral nations ‘‘maintain the tion by the secretary H. G. van de Sande Bakhuyzen existence of the Association under the terms of the old (Comptes Rendus des Se´ ances de la dix-septie` me convention ... for a period that cannot be precisely Confe´ rence Ge´ ne´ rale de l’Association Ge´ ode´ sique defined at present’’. As a consequence, the Reduced Internationale re´ unie a` Hambourg du 17 au 27 Geodetic Association among Neutral Nations was Septembre 1912, Ier Volume, pp. 76–85, Georg formed (Denmark, The Netherlands, Norway, Spain, Reimer, Berlin 1913) and on the activities of the Sweden, Switzerland, and the United States until its Central Bureau by Helmert (pp 129–164), and the entry into the First World War in 1917). summarizing paper by Helmert (1913), while the first The Central Bureau continued to operate, and re- 100 years of the IAG are reviewed by Hunger (1962) ceived data through the Secretary. The main task of this and Tardi (1963). new organization was the continuation of the Interna- tional Latitude Service, but some limited specialized research could also be carried out, such as on Earth References tides, gravity field features and isostatic reductions. After the First World War, the various Scientific Unions Baeyer JJ (1861) U¨ ber die Gro¨ ße und Figur der Erde, eine were created between 1918 and 1920, among them the Denkschrift zur Begru¨ ndung einer mittel-europa¨ ischen International Union of Geodesy and Geophysics Gradmessung. G. Reimer, Berlin (IUGG), founded in 1919. Although this happened Bialas V (1982) Erdgestalt, Kosmologie und Weltanschauung. without any interaction with the countries that had K. Wittwer, Stuttgart formed the Reduced Geodetic Association, the tasks of Bruns H (1878) Die Figur der Erde — Ein Beitrag zur europa¨ ischen Gradmessung. P. Stankiewicz, Berlin the ‘Internationale Erdmessung’ were in practice con- Buschmann E (ed) (1994) Aus Leben und Werk von Johann Jacob tinued and extended, although the loosers of the war Baeyer. Institut fu¨ r Angewandte Geoda¨ sie, Frankfurt am Main remained excluded for a long time, contrary to the Dick WR (1994) Die Vorgeschichte von Johann Jacob Baeyers procedure after the Second World War. At the first ‘Entwurf zu einer Mitteleuropa¨ ischen Gradmessung’. In: IUGG General Assembly in Rome in 1922, a Section of Buschmann E (ed) Aus Leben und Werk von Johann Jacob Geodesy was constituted as part of the IUGG, with Baeyer. Institut fu¨ r Angewandte Geoda¨ sie, Frankfurt am Main, pp 105–144 W. Bowie, Director of the Geodetic Section of the Dick WR (1996) Zur Vorgeschichte der Mitteleuropa¨ ischen Grad- United States Coast and Geodetic Survey, as its first messung. In: Beitra¨ ge zum J.J. Baeyer-Symposium, Deutsche President, acting until 1933. Geod. Komm., Reihe E, Nr. 25: 15–27, Frankfurt am Main With this inclusion into a more interdisciplinary Ekman M, Ma¨ kinen J (1998) An analysis of the first gravimetric body, and the change to a non-governmental organiza- investigations of the Earth’s flattening and interior using tion, the next epoch of the organized international col- Clairaut’s theorem. Small Publ Historical Geophysics, no. 4. Summer Institute for Historical Geophysics, A˚ lands Islands laboration in geodesy started. In 1932, the organization Gauss CF (1828) Bestimmung des Breitenunterschiedes Zurischen officially adopted the name ‘International Association den Sternwarten von Go¨ ttingen und Altona durch Beobach- of Geodesy’ (IAG). More than 140 years after the start tungan_ am Ramsdenschen Zenithsector. Carl Friedrich Gauss of organized cooperation, the IAG is still very proud of Werke Bd. IX: 50. Teubner, Leipzig (1903) this first epoch, which laid the fundamentals for so many Helmert FR (1913) Die Internationale Erdmessung in den ersten research activities, and demonstrated the transition from fu¨ nfzig Jahren ihres Bestehens. Int Monatsschr Wiss Kunst Tech 7(4):397–424 a research project to an international organization Ho¨ pfner J (2000) The International Latitude Service — a historical providing a multitude of results and services for the review, from the beginning to its foundation in 1899 and the different demands of science and geodetic practice. period until 1922. Surv in Geophys 21:521–566 Hunger F (1962) Hundert Jahre Internationale Erdmessung. Z Vermess 87:117–125 Lambert WD (1950) The International Geodetic Association (Die Postscript Internationale Erdmessung) and its predecessors. Bull Ge´ od 17:299–324 The conferences and the results of the forerunner Levallois JJ (1980) The history of the International Association of organizations of IAG are well documented in Geodesy. In: The Geodesist’s Handbook 1980. Bull Ge´ od ‘Verhandlungen der Allgemeinen Conferenz der Inter- 54:249–313 nationalen Erdmessung und deren Permanenten Com- Perrier G (1939) Petite Histoire de la Ge´ ode´ sie. Comment l’homme a mesure´ et pese´ la Terre. Alcan, Presses Universitaires de mission (Comptes-Rendus de la Confe´ rence Ge´ ne´ rale France, Paris [German translation by E Gigas (1950) Wie der de l’Association Ge´ ode´ sique Internationale et de sa Mensch die Erde gemessen und gewogen hat — Kurze Ges- Commission Permanente)’ and in the ‘Publikationen chichte der Geoda¨ sie. Bamberger Verlagshaus Meisenbach, des Ko¨ nigl. Preuss. Geoda¨ tischen Institutes, Berlin’. A Bamberg] summary of the meetings between 1861 and 1880 Sadebeck M (1883) Register der Protokolle, Verhandlungen und is given by Sadebeck (1883). These documents are Generalberichte fu¨ r die Europa¨ ische Gradmessung vom Jahre 1861 bis zum Jahre 1880. Publ des Ko¨ nigl Preussischen Geo- available at the Geoforschungszentrum Potsdam da¨ tischen Instituts, Berlin (GFZ), the support of which is gratefully acknowl- Tardi P (1963) Hundert Jahre Internationale Erdmessung. Z Ver- edged. On the 50th anniversary of the foundation of mess 88:2–10 568

Torge W (1994) Die Geoda¨ sie im U¨ bergang zur international Vermessungsgeschichte in Dortmund 1996. K Wittwer, Stutt- organisierten Wissenschaft: Zum 200. Geburtstag von Johann gart, pp 39–65 Jacob Baeyer. Z Vermess 119:513–522 Torge W (2001) Geodesy, 3rd edn. W de Gruyter, Berlin–New york Torge W (1996) The International Association of Geodesy (IAG) — Torge W (2002) Mu¨ fflings geoda¨ tisches Wirken in der Umbruc- More than 130 years of international cooperation. J Geod hepoche vom 18. zum 19. Jahrhundert. Z Vermess 127:97–108 70:840–845 Vo¨ lter U (1963) Geschichte und Bedeutung der Internationalen Torge W (1997) Von Gauß zu Baeyer und Helmert — Fru¨ he Ideen Erdmessung. Deutsche Geod Komm. Reihe C, Nr. 63, Mu¨ nchen und Iniativen zu einer europa¨ ischen Geoda¨ sie. In: Junius H, Wolf H (1993) Friedrich Robert Helmert — sein Leben und Wir- Kro¨ ger K (eds) Europa wa¨ chst zusammen — 6. Symposium zur ken. Z Vermess 118:582–590